Home-Made Digital Clock Keeps on Ticking

It's hardly an MP3 player or a cell phone, but the digital clock I built while in college still has life in it. It's 27 years old and it probably will keep time long after today's gadgets rot in landfills.

The clock I built uses a National Semiconductor MM5316 clock chip mounted on a 40-pin wire-wrap DIP socket. (Download a data sheet.) The MM5316 is a remarkable chip—it drives LEDs directly, with no need for additional transistors or additional power. It has a relay that I used to turn off an external radio plugged into an AC mains socket on the clock's rear panel. Front-panel buttons (see photo) let me view seconds and let me set the time. Toggle switches control the sleep timer and accessory. The accessory switch forces the relay on, letting power reach the AC socket.

I used the clock to wake me every morning until about a year ago, when its piezoelectric buzzer lost its volume. A 2N2222 transistor drives the buzzer from a pin on the MM5316. Armed with a DMM, I traced the signal back to the pin and found that it no longer pulled high (to 12 V) when it should. Rather than look for another MM5316, I bought an alarm clock for $7, less than I paid for the parts in 1978.

Not wanting to throw away the clock, I brought it to work where it still displays the time. As you can see from the photo, the LEDs are somewhat dim now and are difficult to read under the office lights. No problem, I simply put a small cardboard box around them, which blocks enough light to make them visible. (I adjusted the brightness and contrast of the photo so you could see the LEDs without the box.)

I bought the MM5316 for $3 at a surplus electronics store called Verada 214, located in a former textile mill in Lowell, MA. The LEDs, buzzer, relay, and several other components also came from the store. Verada 214 is long gone, but the building still stands. The former parking lot is now a summer concert venue.

Do you have home-made electronics gadgets? We'd like to hear about them, send e-mail to Senior Technical Editor Martin Rowe at martin.rowe@ubm.com.